Many types of present day power supplies employ a transformer having a primary winding which receives an alternating current from a semiconductor power switching device. The transformer also has a secondary winding coupled to rectifying and filtering circuitry for delivering one or more D.C. voltages to a load. When such a power supply includes a high voltage, high speed switching transistor, it is necessary to accurately control the turn-on and turn-off of the transistor for optimum operation. To be operated near its rating, a high voltage fast-acting transistor needs a highly controlled base drive, and the saturation of the transistor must be controlled. If the transistor is not saturated enough, there are conduction losses, and, if it is saturated too much, there are excess charges in the base which must be removed to turn off the device.
The prior art provides no completely satisfactory circuit for operating a power switching transistor to obtain optimum operation and to avoid the problems described above.
The present invention provides a circuit for achieving optimum control of the base current of a power transistor in a power supply and for thereby achieving fast turn-on and turn-off of the transistor.